1. Field of the Invention
The present invention relates generally to ballasts for gaseous-discharge lamps, such as fluorescent lamps, and more specifically to a dimmable, high power factor, high efficiency electronic ballast with a single integrated circuit semiconductor controller chip.
2. Description of the Prior Art
Incandescent lamps have provided electric light for streets, homes and offices for over a century. They provide a safe light that has no open flame, and produce no soot as did their predecessor gas lamps. More efficient, albeit initially more expensive, lighting systems are now supplanting incandescent lamps. Fluorescent lamps, a type of gas discharge lamp, require about one-third, or less, the electrical power of an incandescent lamp to produce the same light output.
Unlike incandescent lamps which are self-limiting as a result of their positive-resistance characteristic, gaseous-discharge lamps have a negative-resistance characteristic and are not self-limiting. For this reason, gaseous-discharge lamps are operated in conjunction with a ballast which provides the requisite current limiting. Traditionally, the construction of ballasts have included cores and coils. One form is that of a simple choke which provides an inductive impedance for current limiting. Another form includes a transformer. The transformer form permits voltage conditioning and provides a high break-down potential which is required for starting most fluorescent lamps by ionizing the enclosed gas to a plasma. For rapid-start-type fluorescent lamps, a pair of windings are included in the transformer for energizing the lamp filaments and, separate from the filaments windings, a high-voltage winding having a high reactance for current limiting. Alternatively, a magnetic shunt may be included in the transformer to limit the energy transferred through the magnetic path.
Unfortunately, traditional core-and-coil-type ballasts are relatively inefficient due to substantial heat generating losses that are generally equally divided between copper losses in the coil and core losses in the relatively inexpensive grades of iron employed therein. For example, it is not unusual for a traditional core-and-coil-type ballast employed in a dual forty-watt lamp fixture to dissipate from fifteen to twenty watts, causing the ballast to run hot to the touch. Further, in many applications, such as in office buildings, this ballast-generated heat must be removed by air conditioning equipment which is itself relatively inefficient. Another problem is that core-and-coil-type ballasts are relatively heavy requiring that associated fixtures be more substantial than would otherwise be necessary.
The regulation afforded by traditional core-and-coil type ballasts is also relatively poor. Typically, the operating level of fluorescent fixtures employing such ballasts varies as much as the square of the power-line voltage. Thus, in many applications, excessive lighting, dissipating excessive power, is often employed to insure that minimum lighting levels are achieved.
Among other problems associated with gaseous-discharge lamps is that they are less efficient when operated at the normal sixty Hz line frequency than when they are operated at higher frequencies. Fluorescent lamps are often difficult to start when cold and, as a result, may flicker for a time. Fluorescent lamps require core-and-coil-ballast lead-lag phasing both to reduce stroboscopic effects and to increase the power factor such lamps present to the line via the ballast.
Electronic ballasts and networks for gaseous discharge lamps are described in the following United States Patents which were issued to the present inventor, Ronald A. Lesea: U.S. Pat. No. 4,415,839, issued Nov. 15, 1983; U.S. Pat. No. 5,047,691, issued Sep. 10, 1991; and U.S. Pat. No. 5,101,140, issued Mar. 31, 1992.
Gaseous discharge lighting systems can load a commercial AC electrical power supply in such a way that current demand is increased and distorts an otherwise sinusoidal voltage waveform. Watt meters, used for billing purposes, do not see this increase, and the load inappropriately burdens the electric company supplying power. Other users on the same grid can be adversely affected by the distorted waveform, and the problem becomes very serious when such lighting systems are used exclusively in large high-rise office buildings. It has therefore become a requirement of ballasts to include some power factor correction (PFC). Such PFC should preferably also include a means for eliminating any potential runaway condition that can result during load removal, and have low total harmonic distortion (THD).
It is common to install wall dimmers that work with ordinary incandescent lamps. However, ordinary wall dimmers do not properly function with ordinary fluorescent lights. In fact, many fluorescent light fixtures structured to screw directly into a socket for a 120 volt household incandescent light carry consumer warnings not to use the fluorescent lamp with the dimmer. Dimmable fluorescent lights are not familiar to most consumers, even though such systems have been available from limited sources in the last few years.
One significant advantage of fluorescent lamps is their comparative long life. The long life is a substantial factor in the computation of whether or not fluorescent lighting systems are cost effective compared to ordinary incandescent lights. The cost of a fluorescent lamp tube is much higher than that of an incandescent lamp bulb. It is therefore imperative that a ballast does not reduce the intrinsic life of the fluorescent tubes it powers. One way to do that is to properly heat the filaments in the ends of the fluorescent tubes before and during the application of high voltage for starting.
A particular difficulty to consumers, presented with the failure of a fluorescent lamp tube, is determining which tube has failed. Many ballasts are such that if one tube fails, all the tubes go dark. Compounding the problem, some ballasts maintain the application of full power and do not properly manage filament power. What started as a simple failure of one tube, can compound into the premature failure of one or more other tubes in the system. Therefore, a ballast is now needed in the industry that protects the remaining good tubes when only one tube fails.
The heat that has been traditionally generated by ballasts has prompted various standards testing and underwriting laboratories to issue a requirement that ballasts have some kind of over-temperature shut-down. A prior art approach to sense an over-temperature condition of the ambient within a ballast has been to use a separate heat sensor and controller. What is needed in order to reduce costs and improve reliability is an integrated, on-board heat sensor that senses the heat of the ambient that soaks into the controller.